The present invention relates to an electrical circuit, and more particularly a non-linear transimpedance amplifier for use in a detection system. The present invention also relates to a method of changing the gain of an electrical circuit.
The existence of a fire, for example, may be detected by various means, and it is known to employ a plurality of sensing elements that are responsive to different parameters. One such sensing element may be a carbon monoxide gas detector. In some installations there is a requirement also to detect toxic gases, of which carbon monoxide may be one. Conventionally, separate fire and gas detection circuits are installed each having, for example, its own carbon monoxide sensing element. The fire and gas detection circuits are arranged to provide, separately, the required measurement accuracy. The fire circuit and sensing element are sensitive to, say, a range of 0 to 40 parts per million (PPM) and the gas detection circuit and sensing element are sensitive to, say, a range of 35 to 500 PPM.
It is advantageous, in some instances, to have an arrangement of a single sensor and detector for detecting both fire and toxic levels of gas. However, by simply replacing two sensors and their respective detectors with a single sensor and detector, accuracy of measurement over a range of, say, 0 to 500 PPM may not be sufficiently accurate. This is particularly true for measurements at the lower end of the range, which are generally used to detect the existence of fire. For example, when low output levels are sent from a detector to an analogue-to-digital converter (ADC) or comparator having a limited resolution, the quantisation effect could result in the signal being rounded to levels that are too coarse to be of use.
A known approach to solving this problem is to use a non-linear device within a detector. In this way, the gain of the detector may be high when a small input signal is present and low when a high input signal is present. Examples of non-linear devices include diodes, varistors and Zener diodes. However, these devices have undesirable characteristics that result in unreliable or unpredictable operation, including temperature dependency, leakage current, noise and tolerance. In addition, U.S. Pat. No. 5,061,865 discloses a complex solution to the problem in the form of a non-linear transimpedance amplifier comprising an accumulator, a comparator, a sample and hold and a summer. This is not an ideal solution to the problem due to factors such as component cost, component count and power consumption.
It is an aim of the present invention to provide an electrical circuit having a non-linear response, or transfer charcteristic. The circuit finds application, for example, in a detection system.